1. Field of the Invention
This invention relates to handdown and handoff procedures for mobile stations traveling between adjacent or overlapping cells of wireless communication systems. In particular, the invention provides handdown and handoff procedures which are triggered on the basis of propagation path loss between a mobile station and a base station currently serving the mobile station.
2. Discussion of the Known Art
In addition to signals carrying voice traffic, a base station of a cellular wireless communication system transmits at least one control signal at a known power level over its geographic area of coverage or xe2x80x9ccellxe2x80x9d. For example, a base station of a code-division multiple access (CDMA) system radiates a steady pilot signal having a repetitive pseudo random binary sequence code. The pilot signals of all base stations of a given CDMA system have the same binary sequence code, but have different time offsets relative to a zero time reference. When received by a mobile station, the pilot signals allow the mobile station to obtain initial system synchronization, and to link with a system base station whose received pilot signal is strongest among other received pilot signals. The pilot signal also provides a code that the mobile station uses to decode other signals from the system base stations, namely, synchronization (sync), paging, and traffic channels.
Base stations of a time-division multiple access (TDMA) system and of a frequency-division multiple access (FDMA or xe2x80x9canalogxe2x80x9d) system, also transmit steady control signals at known power levels to mobile stations traveling in the base station cells over forward control frequency channels. In TDMA systems such as, e.g., American Digital Cellular (ADC), the global system for mobile communications (GSM), and Japanese digital cellular (JDC), such signals include synchronization (SYNC), slow associated control channel (SACCH) and digital verification color code (DVCCH) signals. In analog systems such as, e.g., the advanced mobile phone system (AMPS), each base station transmits a continuous supervisory audio tone (SAT) control signal for reception by mobile stations in the base station""s cell. See generally, R. C. V. Macario, Cellular Radio Principles and Design (McGraw-Hill 1993); and R. Kuruppillai, et al., Wireless PCS (McGraw-Hill 1997).
In typical cellular wireless communication systems, a mobile station within a system cell is linked by a serving base station for two-way communication with a public switched telephone network (PSTN) or mobile switching center (MSC). The system base stations are themselves connected by wire to the mobile switching center. The MSC interfaces user traffic over wireless links between the base and the mobile stations, with the wired PSTN. An important function of the MSC is to ensure that a mobile station""s link with the PSTN meets a minimum quality standard as the mobile station travels and signaling (i.e., propagation) conditions between the mobile station and the serving base station vary accordingly. The MSC will therefore operate to switch a mobile station to service by another base station, whenever a quality link with a currently serving base station becomes impossible to maintain.
If a mobile station is being served by a first base station affiliated with a first communication system (e.g., a CDMA system using a first set of frequency channels (F1)), and that base station""s cell borders on a cell of a second base station affiliated with a second communication system (e.g., FDMA such as Advanced Mobile Phone System or AMPS, or CDMA using a different set of frequency channels (F2)), a xe2x80x9chardxe2x80x9d handoff of the mobile station to service by the second base station must occur as the mobile station approaches the latter and moves out of range of the first base station. Otherwise, the mobile station will lose its link with the PSTN (a so-called xe2x80x9cdroppedxe2x80x9d call). A hard handoff can be carried out directly, i.e., the mobile station is switched over directly for service by the second base station; or indirectly via an intermediate xe2x80x9chanddownxe2x80x9d procedure wherein the currently serving base station begins to serve the mobile station using the operating protocols of a second communication system (for example, the serving base station hands the mobile station down from CDMA to AMPS).
In CDMA systems, a known method of determining when a hard hand-off or handdown is necessary involves measuring received pilot signal strength in the form of a ratio Ec/Io at the mobile station, wherein Ec is received pilot signal power and Io is total received signal power at the mobile station, and initiating a hard handoff or handdown when the measured Ec/Io ratio falls below a set threshold. Currently, CDMA mobile stations measure Ec/Io and transmit a corresponding pilot strength measurement message (PSMM) based on the Ec/Io measurements to a serving base station, either in response to pilot strength measurement request orders (PMROs) from the base station, or if certain handoff trigger thresholds are met. Although CDMA base stations periodically request PSMMs from mobile stations they currently serve, such requests and the responses usually do not result in handoffs of the mobile stations. These ongoing signal exchanges occur irrespective of whether or not the trigger threshold is met, and increase processor loading of the system:infrastructure thus tending to degrade the voice quality of existing calls.
Using Ec/Io measurements to trigger a handdown and/or handoff incur the following problems, however.
1. The total receive power Io at the position of the mobile station is a function of cell loading, which condition typically varies over time. Using a trigger threshold based on the Ec/Io ratio at the mobile station thus makes the threshold sensitive to the current traffic load condition at the serving base station. Accordingly, a handoff trigger based on received Ec/Io does not accurately reflect the ability of the serving base station to sustain a quality voice link with a given mobile station.
2. Using set handoff or handdown trigger thresholds based on Ec/Io can cause a handoff either too soon or too late, because the Ec/Io measurements with which the thresholds are compared vary depending on traffic loading of the system. Setting the thresholds too high will cause a handoff to be initiated too soon when the mobile station travels inward (i.e., toward) the serving base station, while setting these thresholds lower can seriously delay a handoff or handdown as the mobile station moves farther into the bordering cell of the second communication system. Also, triggering a handoff or handdown too early causes the base station cell to reduce its traffic capacity unnecessarily. On the other hand, a late handoff or handdown impairs the quality of an existing voice link.
3. In an analog system such as AMPS, a typical base station covers less area than a typical CDMA base station. In the region of a strong CDMA pilot signal due to light traffic load, the pilot coverage for a CDMA base station is expanded because the received Ec/Io ratio at the mobile station increases. Thus, if the base station attempts to handdown the mobile station for AMPS service by the same base station in such a region, then the call may be dropped because the station""s AMPS coverage is too small when compared to its CDMA pilot coverage.
A scheme is provided for triggering a handoff or a handdown of a mobile station served by a base station in a cellular wireless communication system. Propagation path loss for signal links between the base station and the mobile station is used to trigger a handoff or a handdown. For example, the scheme can include determining a tolerable path loss for signal links between the mobile and the base stations, and radiating a control signal at a known transmit power level from the base station over its associated cell. A receive power level threshold for the control signal is determined for the mobile station, according to the known transmit power level of the control signal and the tolerable path loss. A handdown or a handoff of the mobile station is triggered after deriving the received power level of the control signal at the mobile station, and determining that the received power level is less than the receive power level threshold.
According to another aspect of the invention, a method of triggering a handdown or a handoff of a mobile station served by a base station of a cellular wireless communication system, includes receiving, at the mobile station, a control signal transmitted at a known power level from the base station, and a receive power level threshold for the control signal which threshold corresponds to the known transmit power level of the control signal and a tolerable path loss for signal links between the mobile and the base stations. A handdown or a handoff of the mobile station is triggered after the mobile station derives the received power level of the control signal, and determines that the received power level is less than the receive power level threshold.
For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawing and the appended claims.